First-time visitors to churches across the country say churches with 300 to 500 attendees perform the best when it comes to first impressions, according to a recent report. Their experiences at mega churches and smaller-sized congregations weren't quite as enjoyable.

Faith Perceptions partnered with church leaders and sent out "mystery guests" to rate their first-time visits at churches varying in size from dozens of members to over a thousand. They found that "large" churches – with average weekly attendance of 300-500 – ranked "best in category" in nine out of 15 areas.

Such churches did particularly well with pre-service greetings, pre-service atmosphere, seating, in-service greetings, friendliness, directions and signage, according to how the visitors rated them. The overall experience was good enough that they gave a 7.48 score (10 point scale) on whether they would return – the highest score compared to other churches.

According to the scale used by Faith Perceptions, a 7.01-7.50 is marked as "fair," a 7.51-8 is "good" and over 8 is "very good."

But there were still some areas that the "large" churches ranked lower in compared to others. The only "poor" or "very poor" score that "large" churches received was with regard to its children's ministries.

First-time guests, meanwhile, found children's ministries to be the best at mega churches (defined as having at least 1,000 attendees). Mega churches also had the highest scores in the area of sermon, speaker and connect-resources.

What mega churches ranked poorly in, however, was the "post-service atmosphere" with a score of 4.73. And that's where micro congregations (with 0-80 in attendance) did best with a score of 7.88.

In terms of overall experience and whether one would return, micro churches ranked lowest.

Mystery guests surveyed 1,152 separate and unique church services using identical questions and criteria between 2008 and July 2012. The Faith Perceptions Church Index is designed for church leaders who want to understand how their church impacts first-time visitors.

According to Faith Perceptions, church leaders typically do not disclose their participation in the program to the congregation so that everyone will conduct "church as usual."

Mystery guests are found through advertising in a church's local market and paid for each completed survey. They are generally unchurched. Churches that participate have at least 12 of its services attended by mystery guests (each service is attended by a different individual).

For this report, 108 churches were surveyed and a majority of the services were traditional while nearly a third were contemporary. Ninety-one percent of the mystery guests believe in God or some higher power and the average age is 41.

Faith Perceptions began in 2008 as it adapted its mystery shopping program for businesses to the church environment using unchurched mystery guests.

Currently, the firm has approximately 4,000 mystery guests in 43 states.